This is what I see in the ADMapping.daml.
<splitButton id="esri_mapping_locateSplitButton" keytip="L" extendedCaption="Search for addresses">
<button refID="esri_mapping_showLocateDockPane"/>
<gallery refID="esri_mapping_locateGallery"/>
</splitButton>
So, I think if u reference the locateSplitButton rather than the showLocateDockPane u will be good to go.